The present disclosure relates to a sheet post-processor that forms punch holes through a sheet such as a paper sheet on which an image has been formed by an image forming apparatus such as a copy machine or a printer, and an image forming system provided with the same.
Conventionally, there has been used a paper sheet post-processor that is capable of executing post-processing such as a binding process of stacking a plurality of paper sheets (sheets) on which images have been formed by an image forming apparatus such as a copy machine or a printer and binding together a bundle of the paper sheets thus stacked with a staple(s) and a punch hole formation process of forming punch holes (perforations) through the bundle of the paper sheets by using a punch unit.
As such a paper sheet post-processor, there is known a paper sheet post-processor provided with a punch unit that is capable of forming punch holes along a first side end edge of a paper sheet parallel to a paper sheet width direction orthogonal to a paper sheet conveyance direction and a second side end edge of the paper sheet parallel to the paper sheet conveyance direction and moves in the paper sheet width direction, an edge sensor that moves in the paper sheet width direction together with the punch unit and detects the second side end edge of the paper sheet, and a registration roller that is disposed downstream of the punch unit in the paper sheet conveyance direction and corrects skewing of the paper sheet by forming a warp in the paper sheet.
In this type of paper sheet post-processor, in a case of forming punch holes along the first side end edge of a paper sheet, after skewing of the paper sheet has been corrected by the registration roller, the punch unit and the edge sensor move in the paper sheet width direction, and the edge sensor detects the second side end edge of the paper sheet, while the punch unit forms punch holes through the paper sheet.
In a case of forming punch holes along the second side end edge of a paper sheet, however, there is a problem with the configuration in which, after skewing of the paper sheet has been corrected by the registration roller, the punch unit and the edge sensor move in the paper sheet width direction, and the edge sensor detects the second side end edge of the paper sheet. Specifically, in this configuration, in a case where a distance from a tip end of a paper sheet to a position on the paper sheet at which a first punch hole is to be formed is short, the punch unit and the edge sensor might fail to be properly displaced in time. That is, the punch unit and the edge sensor move in the paper sheet width direction, and the edge sensor detects the second side end edge of the paper sheet, while the punch unit moves to a prescribed position in the paper sheet width direction; before this happens, however, the position on the paper sheet at which the first punch hole is to be formed might reach the punch unit. To avoid this, it is required to stop the paper sheet from being conveyed for a duration longer than necessary, resulting in a decrease in productivity (the number of sheets processed per unit time). For this reason, in the case of forming punch holes along the second side end edge of a paper sheet, after the punch unit and the edge sensor have moved in the paper sheet width direction, and the edge sensor has detected the second side end edge of the paper sheet, skewing of the paper sheet is corrected by the registration roller, and the punch unit forms punch holes through the paper sheet.